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LMP8646: Use with TPS61087

D House
Intellectual 275 points
Part Number: LMP8646
Other Parts Discussed in Thread: TPS61087, , INA169, INA240, INA193

Greetings,

Is the LMP8646 suitable for use with TPS61087 boost regulator for precision current limiting application?

  • 0
    TI__Mastermind 31165 points
    Hello D House,

    It certainly looks possible, but it really comes down to your design requirements. What is your limit (closed-loop current) and open-loop current? Are you driving a capacitive or resistive load? You can find these design requirements and calculations in Section 8 of LMP8646 datasheet.

    Sincerely,
    Peter Iliya
    Current Sensing Applications
  • 0 in reply to Peter Iliya
    Intellectual 275 points
    I am driving a complex load (H-bridge driving a coil). Input capacitance on the order of 100uF ceramic, max. Mostly the current output of the boost will be low frequency/DC.

    The closed current limit would be on the order of 500 mA to 1 A (TBD). I'm not sure what is means by open-loop current but if its before the current limit is hit then the current would be nominally 250mA (with inrushes on startup).
  • 0 in reply to Peter Iliya
    Intellectual 275 points
    Are there any alternative parts to LMP8646 that serve the same purpose?
  • 0 in reply to D House
    TI__Mastermind 31165 points
    Hey D House,

    To answer you earlier question, the open-loop current should be the current level below current limit threshold. Thus I believe at this current the current sensor/limiter should not be driving the FB node with a higher than normal voltage.

    Technically many of the analog/voltage-output current sensors can serve this purpose of current limiting a power supply. The LMP8646 has the extra benefits of variable gain and bandwidth, as well as, a output buffer with an output diode for protection I assume.

    Some options would be the current-output INA169. You would need an operational amplfier buffer to drive the FB pin, but INA169 would provide variable gain, high-common mode voltage range, and decent bandwidth. I should note that the gain setting resistor determines BW, while for LMP8646, the bandwidth is determined if you provide a capacitor in parallel with RG.

    The INA193 has a buffered output, but not variable gain. The INA240 could be a good option because it has a high common-mode range (80V), but also has very good common-mode rejection ratio (CMRR) and good bandwidth. The CMRR would be helpful if the voltage output of TPS61087 is noisy and contains switching voltage spikes that the INA240 can reject very well. This is why we recommend the INA240 for in-line motor control where the common-mode can switch back and forth.

    Hope this helps,
    Peter Iliya
    Current Sensing Applications